Try this...

1. Unplug PIC from board.
2. Measure volts on socket at RA4. You should have what you expect to see from your sensor. This signal should be the same at RA4, J3.1 and J3.2. If it isn't, you have a problem.

3. Plug PIC back into board... voltage at RA4 should be what you had before. This signal should be the same at J3.1

If there is a difference between the PIC being plugged-in or not, then...

A. The PICs software is not configured for INPUT proprerly
B. There is a defect with the PIC
C. The Sensor is defective or otherwise has insufficient drive for the PIC.

I have assumed that the Return for the Sensor is part of the Engine GROUND, and you have also connected 0v on your board to Engine GROUND.